European Journal of Orthodontics 28 (2006) 426–432
doi:10.1093/ejo/cjl002
Advance Access publication 11 August 2006
© The Author 2006. Published by Oxford University Press on behalf of the European Orthodontic Society.
All rights reserved.
Toxicity of used orthodontic archwires assessed by
three-dimensional cell culture
Bart Vande Vannet*, Nahid Mohebbian** and Heinrich Wehrbein***
Departments of *Orthodontics and **Restorative Dentistry, Free University of Brussels, Belgium,
***Department of Orthodontics, University of Mainz, Germany
The aim of the present study was to determine whether used orthodontic wires made of
different materials cause toxicity and loss of viability on three-dimensional (3D) cell cultures. Three
types of orthodontic wires, stainless steel, Nitinol®, and TMA® (n = 9) which had been used clinically in
ﬁxed appliances for a period of 1 month, were retrieved at random from ﬁve patients. Both upper and
lower archwires were collected and subjected to two different protocols: to assess toxicity, two pieces of
each wire were placed on 3D cell cultures (reconstituted human epithelium); to investigate the possibility
of cell damage, the 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl tetrazolium bromide (MTT) assay was used
and haematoxylin and eosin staining was performed to evaluate morphological changes. Copper wire
served as the control to determine the morphology of severe toxicity, and native cell cultures and silk
were used as the negative controls.
Morphological evaluation of the native cell cultures revealed no toxic reactions. The ranking, from mild
to severe toxicity was as follows: stainless steel < Nitinol® = TMA®. There were no signiﬁcant differences
between TMA® and Nitinol®. The MTT assay revealed the following mean percentage values for viability:
native cell line (negative control), 100; stainless steel, 102.25; TMA®, 87.4; Nitinol®, 85.3; and copper wire
(positive control) 57.2. Histological evaluation of the 3D cell cultures showed no severe toxicity or loss
of viability for any of the wires. However, relative comparison between the different wires revealed that
stainless steel induced less toxicity/loss of viability compared with TMA® and Nitinol® wire.
SUMMARY
It is known that orthodontic materials, such as archwires,
may cause adverse clinical effects (Grimsdottir and HenstenPettersen, 1993). Clinical use of orthodontic appliances is
often associated with an increase in gingivitis, most often
related to oral hygiene (Zachrisson and Zachrisson, 1972).
Intraoral reactions, such as redness, soreness, and swelling
of the oral mucosa, gingiva, and/or lips, have been associated
with metal brackets, labial wires, bonding procedures, and
acrylic appliances (Jacobson and Hensten-Pettersen, 1989).
In some instances, it may be that the appliances or their
corrosion products can cause local tissue damage, which
cannot clinically be distinguished from gingivitis of a
bacteriological aetiology. For a material to be biocompatible,
it must exert no toxic effects on the organism. Estimation of
cytotoxicity is part of the initial evaluation of biocompatibility (Babich and Sinensky, 2001).
Humans are habitually exposed to a large variety of
foreign substances that are potentially toxic and harmful to
different organs and tissues. Substances capable of producing
cell damage are known as ‘toxins’ and are classified according
to whether they exert their effects in all individuals, in a
dose-dependent and predictable manner (intrinsic toxins),
or only in some individuals, usually after several contacts, in
a non-dose-dependent and therefore unpredictable way
(idiosyncratic toxins). Intrinsic toxins may act directly on
cellular systems (active toxins) or after biotransformation by
hepatocytes (latent toxins). Idiosyncratic toxicity may be the
consequence of an unusual metabolism of the drug (metabolic
idiosyncrasy) or may be mediated by the immune system
after repeated previous contacts (sensitization; Pessayre
et al., 1985; Benford and Good, 1987).
Cell culture techniques may be used to assess local
cytotoxic effects of solid materials (Schmalz and Schmalz,
1981). Human cell lines have been utilized for a wide
variety of purposes in medical research and the number
and range available are increasing (Bernard et al., 2002;
De Wever and Charbonnier, 2002). Cell culture methods are
frequently used to test the biological effects (cytotoxicity)
of dental materials, with special emphasis on local irritation
(Ames et al., 1973; Hensten-Pettersen, 1981). Cytotoxicity
has also been assessed using different methods, e.g. counting
surviving cells, measurement of proliferation rates, synthesis
of cellular macromolecules, or determination of enzyme
activity (Ames et al., 1973). The cytotoxicity of the most
common alloys used in orthodontic appliances has been
determined by cell culture testing (Locci et al., 2000). It is
necessary, however, to consider all the relevant information
regarding the derivation of new materials (United States
National Bioethics Advisory Commission, 1999).
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Introduction
427
TOXICITY ASSESSED BY 3D CELL CULTURE
1. The use of a biological system that reproduces, to a large
extent, the metabolic behaviour of the target organ for
the toxic effect of the xenobiotic.
2. The choice of appropriate parameters for evaluating the
toxic effect in vitro.
3. A correct experimental design so that the in vitro data are
predictive of the potential in vivo effects.
Materials and methods
The in vitro model: three-dimensional human
epithelium cell culture
The three-dimensional (3D) human epithelium model for
the in vitro testing of cytotoxicity was supplied by
Skinethic™ Laboratory (Nice, France). The cytotoxic effect
was undertaken on containing reconstituted human oral
epithelium (TR146 cell line) 0.63-cm2 on inert polycarbonate
filters. When cultivated at the air–liquid interface in defined
medium, the transformed human keratinocytes of the cell
line TR146 form an epithelial tissue devoid of stratum
corneum, resembling, histologically, the buccal mucosa.
The in vitro model and all culture media were prepared
without antibiotics and antimycotics. The cell cultures were
controlled for biological safety. On a cell pellet of the same
strain, the absence of HIV-integrated pro-viral DNA, and
hepatitis XC viral DNA was verified as the absence of
cytomegalovirus DNA by polymerase chain reaction
(Amplicor) and mycoplasma by Hoechst stain. On the
supernatant culture, the epithelium was verified for the
absence of hepatitis B antigen Hbs and bacteria and fungus.
All tests were carried out at the Tissue Culture Laboratories
of Skinethic.
Test materials
In order to resemble the clinical situation as closely as
possible, three types of orthodontic wires (Table 1), used in
fixed appliances and potentially xenobiotic, were collected
from five patients [three females and two males, with an age
range of 12–15 years (average 13.2 years)] treated in a
private orthodontic practice. The orthodontic appliances
consisted of four bands and 20 bonded brackets. All wires
(n = 9) were collected at random from the upper and lower
arch after an intraoral exposure period of 1 month. Two
1 mm sections of the wires were cut 5 mm from the distal
Table 1 Used orthodontic wires and metal composition of different components in orthodontic wires expressed in percentage of weight
and rounded to the nearest integer (as provided by the manufacturers).
Product
(manufacturer)
Batch
number
n
Wire size
Weight percentage
Mn
Si
Ni
Ti
Sn
Zr
Mo
Fe
Cr
C
Nitinol® (3M
Unitek, Monrovia,
California, USA)
293-512
3
0.014 inch
—
—
54
46
—
—
—
—
—
0.03
TMA® (OrmcoSDS, Orange,
California, USA)
266-0011
3
0.017 ×
0.025 inch
—
—
—
78
4.5
6
11.5
—
—
—
3
0.016 ×
0.022 inch
2
1
8
—
—
—
—
69
19
0.08
Stainless steel
03-622-63
(S30400; GAC
International, Bohemia, New
York, USA)
Mn, manganese; Si, silicon; Ni, nickel; Ti, titanium; Sn, tin; Zr, zirconium; Mo, molybdenum; Fe, iron; Cr, chromium; C, carbon.
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Grimsdottir and Hensten-Pettersen (1997), in an
optical microscopy study, revealed islands of amorphous
precipitants and accumulated microcrystalline particles on
used orthodontic wires. They found that after 4 weeks the
elemental species precipitated on the material surface were
Na, K, and Cl, and after 3 more months Ca and P forming
NaCl, KCl, and Ca–P precipitates. Intraoral exposure to
orthodontic wires (i.e. NiTi) alters the topography and
structure of the alloy surface through surface attack in the
form of pitting or crevice corrosion and formation of
integuments (Eliades et al., 2000).
To date, all research on cytotoxicity in orthodontics has
been carried out on monolayer cell cultures (Rose et al.,
1998; Tomakidi et al., 2000). The purpose of the present
investigation was to assess the cytotoxic effect of a range
of used archwires, in either direct or indirect contact
with the oral mucosa during orthodontic treatment,
and to quantify this cytotoxic effect and loss of viability
by means of a multilayered human epithelial cell
culture in vitro (Mohebbian, 2001; Mohebbian and
Bottenberg, 2003). The development of in vitro methods
as alternatives to animal experimentation is therefore of
relevance in biomedical research aimed at detecting the
potential toxicity of xenobiotics in humans (HenstenPettersen, 1981).
The quality and specificity of the data generated by
in vitro models depends on the following factors (Jover
et al., 1992; Castell et al., 1997):
428
B. VANDE VANNET ET AL.
end and each piece was disinfected with alcohol (NedalcoDes-O 380, Belgaco NV, Gent, Belgium) before testing.
deviation (SD) of duplicate cultures]. The following formula
applies:
Testing procedure
Percentage of viability = OD(570–690 nm) test product
× 100.
OD(570–690 nm) negative control
In order to verify the results of the MTT assay, histology
is mandatory. The MTT assay is active on all cells without
taking into account possible necrosis of the upper cell
layers. As the in vitro model comprises different cell layers,
MTT assay results have to be controlled (De Wever and
Charbonnier, 2002).
For each of the tested orthodontic wires or controls, at the
end of each test period, the cell cultures were cut in half.
The two treated cultures were fixed in a balanced 10 per
cent formalin solution and later embedded in paraffin.
Vertical sections (4 μm) were stained with haematoxylin
and eosin, and photographed under a light microscope. The
colour photomicrographs of the stained sections were
compared with untreated native cell cultures.
For histopathological interpretation, a scoring system
(Doucet et al., 1988) was established by taking into account the
overall changes in the morphological parameters (Figure 1).
For control of the reliability of the histological findings,
the results were peered by two independent observers
(anatomo-pathologists)
Results
The different cross-sections of the used orthodontic
archwires were evaluated for their capacity to induce
cytopathic effects on human buccal mucosa reconstituted
by in vitro multilayered cell culture of transformed
keratinocytes. The results of the qualitative evaluation of
cell viability are shown in Table 2. Negative control cultures,
as well as stainless steel and silk, were blue and thus were
NI. TMA® and Nitinol® were blue/white after 24 hours
and were labelled as slightly irritant. Positive controls were
blue/white or white, evidence of cell death and VI.
The SD of the different readings (n = 6) in the colourimeter
for the MTT assay was calculated but showed little difference.
The MTT test revealed the following mean percentage
values for viability: native cells (negative control), 100; silk,
95.32; stainless steel, 102.25; TMA®, 87.4; Nitinol®, 85.3;
and copper wire (positive control), 57.22.
No acute toxicity was found between the tested wires.
The eccentricity of the difference was highly significant
(P < 0.05), not only between the copper (positive control)
and stainless steel wire but also between Nitinol® and
TMA® versus stainless steel (Tables 3 and 4).
The peered morphological histological findings, using the
histological index (Figure 1), revealed no toxic reactions.
The ranking of the morphological evaluation from mild to
severe toxicity was stainless steel < Nitinol® < TMA®.
When compared with the negative and positive controls,
only mild changes were observed between the tested wires.
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A multiple end-point analysis was performed. A sample of
the used wire to be tested was deposited directly on the
reconstituted mucosa. Native cells and silk were used as
internal negative controls and copper wire as the positive
control (Schmalz et al., 1997). Triplicate cultures were
incubated at 37°C, 5 per cent CO2 for 24 hours. The
viability of the keratinocytes of the basal cell layer was
evaluated on duplicate cultures and the effect of the tested
product on the cultures’ histology verified on the third
culture. For each condition, at the end of the test period, the
treated cultures were rinsed twice with phosphate-buffered
saline solution, and placed in 300 μl of 0.5 mg/ml 3-(4,5dimethylthiazol-2-yl)2,5-diphenyl tetrazolium bromide
(MTT; Mosmann, 1983). Bioassays which monitor changes
in metabolic activity or decreases in cell number are
generally accepted methods to evaluate the adverse effects
of xenobiotics on cells (Schmalz, 1982). Included in this
group of bioassays is the MTT test (Mosmann, 1983;
Tomakidi et al., 2000).
The MTT assay is designed to be used for non-radioactive
spectrophotometric quantification of cell proliferation and
viability in cell populations using the 96-well plate format
(Sjogren et al., 2000). This assay is based on cleavage of
the tetrazolium salt MTT, in the presence of an electroncoupling reagent, by active mitochondria. The waterinsoluble formazan salt produced has to be solubilized in
an additional step. After this incubation period, a waterinsoluble formazan dye is formed. After solubilization, the
formazan dye is quantitated using a scanning multiwell
spectrophotometer: the MTT crystals are extracted by
isopropanol and the optical density (OD) of extracts is
measured at 570 nm (reference filter 690 nm). The results
are expressed as a percentage of viability compared with the
negative control.
For qualitative evaluation of cell viability, the colour of
each culture was noted after 10 minutes and 1, 3, and 24
hours: negative control cultures have to be a dark blue
colour, proof of the cell’s viability [non-irritant (NI)], and
positive control cultures blue/white or white, evidence of
cell death [very irritant (VI)].
For quantitative evaluation of cell viability after an
incubation period of 3 hours in 300 μl of 0.5 mg/ml MTT,
the cultures were placed in 2 ml of isopropanol. Extraction
was performed at 37°C, for a minimum of 1.5 hours, by
gentle shaking. OD was measured on 200 μl of extract at
570 nm (reference filter 690 nm). The percentage of viability
values was calculated as the OD reading of the probe divided
by the OD reading of the negative control multiplied by 100.
The results are thus expressed as a percentage of viability
compared with the negative control [mean ± the standard
429
TOXICITY ASSESSED BY 3D CELL CULTURE
Table 2 Qualitative evaluation of cell viability: negative control cultures have to be a dark blue colour, proof of the cell’s viability
[non-irritant (NI)], and positive control cultures blue/white [very slight irritant (VSI)] or white, evidence of cell death [very irritant (VI)].
Products tested undiluted.
Tested Product
Native cells (−)
Native cells (−)
Silk
Silk
Nitinol®
Nitinol®
TMA®
TMA®
Stainless steel
Stainless steel
Copper (+)
Copper (+)
Colour of cultures
Toxicity
10 minutes
of exposure
1 hour of
exposure
3 hours of
exposure
24 hours
of exposure
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue/white
Blue/white
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
Blue
White
White
Blue
Blue
Blue
Blue
Blue/white
Blue/white
Blue/white
Blue/white
Blue
Blue
White
White
NI
NI
NI
NI
VSI
VSI
VSI
VSI
NI
NI
VI
VI
Table 3 Quantitative cell viability: results of optical densitometry at 570 nm after 24 hours incubation at 37°C with MTT assay.
First reading
Second reading
Third reading
Mean value
Calculated mean (SD)
Native cells (−)
Native cells (−)
Silk
Silk
Nitinol®
Nitinol®
TMA®
TMA®
Stainless steel
Stainless steel
Copper (+)
Copper (+)
0.341
0.396
0.352
0.353
0.317
0.316
0.325
0.312
0.417
0.364
0.182
0.235
0.361
0.423
0.373
0.374
0.334
0.336
0.355
0.344
0.403
0.383
0.201
0.249
0.3620
0.424
0.374
0.373
0.333
0.332
0.350
0.331
0.424
0.368
0.199
0.254
0.354667
0.414333
0.366333
0.366667
0.328000
0.328000
0.343333
0.329000
0.414667
0.371667
0.194000
0.246000
0.3845 (0.058 )
0.3665 (0.015)
0.328 (0.0106)
0.336167 (0.004)
0.393167 (0.034)
0.22 (0.05)
MTT, 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl tetrazolium bromide; SD, standard deviation.
The cell cultures for stainless steel showed the most preserved
architecture. For TMA® and Nitinol®, there was only slight
modification in the architecture (Figure 2a,b,c), although the
appearance of apoptosis was observed (Figure 3).
The results of the MTT assay are presented
diagrammatically in Figure 4.
Discussion
The ultimate advantage of the 3D culture system is to allow
direct topical application of insoluble materials onto a
mucosal surface in order to reproduce, as closely as possible,
the topical application achieved in vivo. Most of the metals
used in the oral cavity can be expected to undergo some type
of corrosion (Grosgogeat et al., 2003; Oh et al., 2004;
Yonekura et al., 2004). Although nickel is not highly toxic,
small amounts can cause allergic reactions (Greppi et al.,
1991; Bishara et al., 1993; Grimsdottir et al., 1994; De Silva
and Doherty, 2000). Compared with stainless steel, TMA®
and Nitinol® showed more toxicity when assessed on in
vitro mucosa. The specific metal responsible for the toxicity
was not determined. In the histopathological analysis of
TMA®, Nitinol®, and stainless steel, some cells showed the
appearance of apoptosis with, at the light microscopic level,
single dispersed cells with condensed eosinophilic cytoplasm
and, at an early stage, masses around the margins of nuclei,
or the entire nucleus appeared deeply basophilic (Figure 3).
Apoptosis is a genetically controlled, metabolically active,
evolutionarily conserved process by which cells self-destruct
(Kerr et al., 1972; Wyllie et al., 1980; Steller, 1995;
Thompson, 1995; Cummings et al., 1997). This is in
agreement with the findings of David and Lobner (2004)
who reported that stainless steel induced apoptosis.
The used orthodontic wires were obtained at random
from five different patients and were not preserved in
standard saliva. This means that the used wires, taken crosssectionally, were not standardized or under the same
conditions. Different external attacks, such as food intake
(acids, sugar, etc.), as well as temperature and mechanical
factors, cannot therefore be evaluated. MTT and histology
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Tested materials
430
B. VANDE VANNET ET AL.
Table 4 Quantitative cell viability of tested materials.
Percentage
viability
(OD probe/
Calculated
OD neg × 100) SD percentage mean value SD
Native cells (−) 100.00
Silk
95.32
Nitinol®
85.31
TMA®
87.43
Stainless steel 102.25
Copper(+)
57.22
10.97
0.06
0.00
2.64
7.91
9.56
0.384500
0.366500
0.328000
0.336167
0371667
0.22000
0.35001
0.10858
0.009011
0.009011
0.016388
0.029893
OD, optical density; neg, negative; sd, standard deviation.
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Figure 1 Histological classification index (light microscopy ×40) of
in vitro oral epithelium. (a) The epithelial tissues have a constant thickness,
devoid of terminally differentiated cells, with a regular and compact shape.
Cells are attached to others via multiple desmosomes. (b) Minimal changes
occur with slight oedema. (c) The beginning of spongious tissue
development in the upper layers, with architectural atrophy, and cellular
irregularity. (d) Most of the upper cell layers of the epithelial tissues
become disintegrated, and the remaining basal cells show loose adhering
to the polycarbonate substratum. There is spongious tissue development,
cellular necrosis, and loss of cellular junctions in the basic layer together
with cellular oedema and necrosis in all other cell layers. Some cells appear
to show apoptotic bodies.
Figure 2 Histology results of tested wires (light microscopy ×40)
assessed with classification index (Table 5). (a) Nitinol® (b + c): mild and
moderate tissue disruption. (b) TMA® (b + c): mild and moderate tissue
disruption. (c) Stainless steel (a + b): normal to mild tissue disruption.
were analysed on control (negative and positive) as well as
on the different used wires (n = 9), such that potential
inhomogenicity in one group can be reduced to an acceptable
level using this in vitro model. The MTT assay revealed
102.25 per cent values for viability for stainless steel
431
TOXICITY ASSESSED BY 3D CELL CULTURE
Table 5 Peered assessment of the results of the histological
interpretation using a histological index.
Tested material
A
Native cells (−)
Silk
Nitinol®
TMA®
Stainless steel
Copper (+)
ˉ
ˉ
ˉ
B
C
ˉ
ˉ
ˉ
ˉ
ˉ
ˉ
D
ˉ
(a) The epithelial tissues have a constant thickness, devoid of terminally
differentiated cells, with a regular and compact shape. Cells are attached
to others via multiple desmosomes.
(b) Minimal changes occur with slight oedema.
(c) The beginning of spongious tissue development in the upper layers,
architectural atrophy, and cellular irregularity.
(d) Most of the upper cell layers of the epithelial tissues become
disintegrated, and the remaining basal cells demonstrate loose adherence
to the polycarbonate substratum. There is spongious tissue development,
cellular necrosis, and loss of cellular junctions in the basal layer together
with cellular oedema and necrosis in all other cell layers. Some cells
appear to show apoptotic bodies.
Conclusion
While clinical conditions can never deliver standardized
laboratory pre-conditions, laboratory conditions can never
simulate the full range of different influences in the oral
cavity. Therefore, both clinical and laboratory investigations
are necessary.
Histological evaluation showed no severe toxicity or loss
of viability caused by any of the tested orthodontic wires;
nevertheless relative differences were found.
Histopathological analysis of the positive control and the
used Nitinol® and TMA® wires showed necrosis and
apoptosis. The multiple end-point analysis with comparison
between the different used wires revealed that stainless steel
induced less toxicity/loss of viability compared with
Nitinol® and TMA®. This suggests that stainless steel
wires should be used as soon as biomechanically possible.
None of the examined used orthodontic wires were found
to exhibit acute cytotoxicity, regardless of the type of test
employed.
The human reconstituted epithelium model provides a
constant, stable, quantifiable, and reproducible method for
further in-depth in vitro studies of orthodontic materials.
Figure 3 Histological paraffin section of reconstituted human oral
epithelium stained with haematoxylin and eosin showing the appearance
of apoptotic bodies (magnification ×200).
Acknowledgements
viability of tested materials
120
100
%
80
60
40
20
0
Native
Silk
Nitinol
TMA
Stainless
Steel
Bart Vande Vannet
Department of Orthodontics
Medical and Pharmaceutical Faculty
Free University of Brussels
Laarbeeklaan 103
B-1090 Brussels
Belgium
E-mail: [email protected]
Copper
Figure 4 Viability of tested materials with 3-(4,5-dimethylthiazol2-yl)2,5-diphenyl tetrazolium bromide assay.
archwires compared with untreated tissues (MTT values of
100 per cent). This phenomenon is described as ‘hormesis’
and is a stress response in cells exposed to low levels of
heavy metals (Damelin et al., 2000)
We wish to express our thanks to the Department of
Anatomo-pathology, Academic Hospital [Free University
of Brussels (VUB)], Professor M. Marichal for the histology
study in her department, and Dr A. Goossens for assistance
and interpretation of the histological slides. We would also
like to thank Mr B. De Wever and Dr M. Cappodoro
(Skinethic) and Mrs M. Baekeland (Department of
Cytology-VUB) for their expert technical assistance.
Funding for Open Access publication charges was provided
by Urije Universiteit Brussel (MFOD-SOPA).
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